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    SubjectRe: [PATCH] tracing: add trace_event_read_lock()
    On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote:
    >
    > I found that there is nothing to protect event_hash in
    > ftrace_find_event().



    Actually, rcu protects it, but not enough. We have neither
    synchronize_rcu() nor rcu_read_lock.

    So we protect against concurrent hlist accesses.
    But the event can be removed when a module is unloaded,
    and that can happen between the time we get the event output
    callback and the time we actually use it.


    > RCU can also protect ftrace_find_event(), but it will add
    > latency to kernel. So this fix converts 'trace_event_mutex'
    > to a read/write semaphore, and adds trace_event_read_lock()
    > to protect ftrace_find_event().
    >
    > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
    > ---
    > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    > index 665a915..4ef8052 100644
    > --- a/kernel/trace/trace.c
    > +++ b/kernel/trace/trace.c
    > @@ -1569,12 +1569,14 @@ static void *s_start(struct seq_file *m, loff_t *pos)
    > p = s_next(m, p, &l);
    > }
    >
    > + trace_event_read_lock();
    > return p;
    > }
    >
    > static void s_stop(struct seq_file *m, void *p)
    > {
    > atomic_dec(&trace_record_cmdline_disabled);
    > + trace_event_read_unlock();
    > }
    >
    > static void print_lat_help_header(struct seq_file *m)
    > @@ -1817,6 +1819,7 @@ static int trace_empty(struct trace_iterator *iter)
    > return 1;
    > }
    >
    > +/* Called with trace_event_read_lock() held. */
    > static enum print_line_t print_trace_line(struct trace_iterator *iter)
    > {
    > enum print_line_t ret;
    > @@ -3008,6 +3011,7 @@ waitagain:
    > offsetof(struct trace_iterator, seq));
    > iter->pos = -1;
    >
    > + trace_event_read_lock();
    > while (find_next_entry_inc(iter) != NULL) {
    > enum print_line_t ret;
    > int len = iter->seq.len;
    > @@ -3024,6 +3028,7 @@ waitagain:
    > if (iter->seq.len >= cnt)
    > break;
    > }
    > + trace_event_read_unlock();
    > /* Now copy what we have to the user */
    > sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
    > @@ -3146,6 +3151,8 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
    > goto out_err;
    > }
    >
    > + trace_event_read_lock();
    > +
    > /* Fill as many pages as possible. */
    > for (i = 0, rem = len; i < PIPE_BUFFERS && rem; i++) {
    > pages[i] = alloc_page(GFP_KERNEL);
    > @@ -3168,6 +3175,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
    > trace_seq_init(&iter->seq);
    > }
    >
    > + trace_event_read_unlock();
    > mutex_unlock(&iter->mutex);
    >
    > spd.nr_pages = i;
    > diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
    > index 489c0e8..7136420 100644
    > --- a/kernel/trace/trace_output.c
    > +++ b/kernel/trace/trace_output.c
    > @@ -14,7 +14,7 @@
    > /* must be a power of 2 */
    > #define EVENT_HASHSIZE 128
    >
    > -static DEFINE_MUTEX(trace_event_mutex);
    > +static DECLARE_RWSEM(trace_event_mutex);
    > static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly;
    >
    > static int next_event_type = __TRACE_LAST_TYPE + 1;
    > @@ -466,6 +466,7 @@ static int task_state_char(unsigned long state)
    > * @type: the type of event to look for
    > *
    > * Returns an event of type @type otherwise NULL
    > + * Called with trace_event_read_lock() held.
    > */
    > struct trace_event *ftrace_find_event(int type)
    > {
    > @@ -475,7 +476,7 @@ struct trace_event *ftrace_find_event(int type)
    >
    > key = type & (EVENT_HASHSIZE - 1);
    >
    > - hlist_for_each_entry_rcu(event, n, &event_hash[key], node) {
    > + hlist_for_each_entry(event, n, &event_hash[key], node) {
    > if (event->type == type)
    > return event;
    > }
    > @@ -513,6 +514,16 @@ static int trace_search_list(struct list_head **list)
    > return last + 1;
    > }
    >
    > +void trace_event_read_lock(void)
    > +{
    > + down_read(&trace_event_mutex);
    > +}
    > +
    > +void trace_event_read_unlock(void)
    > +{
    > + up_read(&trace_event_mutex);
    > +}
    > +
    > /**
    > * register_ftrace_event - register output for an event type
    > * @event: the event type to register
    > @@ -533,7 +544,7 @@ int register_ftrace_event(struct trace_event *event)
    > unsigned key;
    > int ret = 0;
    >
    > - mutex_lock(&trace_event_mutex);
    > + down_write(&trace_event_mutex);
    >
    > if (WARN_ON(!event))
    > goto out;
    > @@ -581,11 +592,11 @@ int register_ftrace_event(struct trace_event *event)
    >
    > key = event->type & (EVENT_HASHSIZE - 1);
    >
    > - hlist_add_head_rcu(&event->node, &event_hash[key]);
    > + hlist_add_head(&event->node, &event_hash[key]);
    >
    > ret = event->type;
    > out:
    > - mutex_unlock(&trace_event_mutex);
    > + up_write(&trace_event_mutex);
    >
    > return ret;
    > }
    > @@ -597,10 +608,10 @@ EXPORT_SYMBOL_GPL(register_ftrace_event);
    > */
    > int unregister_ftrace_event(struct trace_event *event)
    > {
    > - mutex_lock(&trace_event_mutex);
    > + down_write(&trace_event_mutex);
    > hlist_del(&event->node);
    > list_del(&event->list);
    > - mutex_unlock(&trace_event_mutex);
    > + up_write(&trace_event_mutex);
    >
    > return 0;
    > }
    > diff --git a/kernel/trace/trace_output.h b/kernel/trace/trace_output.h
    > index 6e220a8..ac240e7 100644
    > --- a/kernel/trace/trace_output.h
    > +++ b/kernel/trace/trace_output.h
    > @@ -20,6 +20,8 @@ extern int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm,
    > extern int trace_print_context(struct trace_iterator *iter);
    > extern int trace_print_lat_context(struct trace_iterator *iter);
    >
    > +extern void trace_event_read_lock(void);
    > +extern void trace_event_read_unlock(void);
    > extern struct trace_event *ftrace_find_event(int type);
    >
    > extern enum print_line_t trace_nop_print(struct trace_iterator *iter,



    It could be more fine grained. We could have a per event rwsem, and also place the
    protected read section only in trace_print_entry() which is the only racy window.

    But I'm not sure it's that worthy since event removal is a rare thing.

    So I guess this patch is fine.



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